A ferroelectric capacitor includes an upper electrode, a lower electrode and a ferroelectric film disposed between the upper and lower electrodes. Such a ferroelectric capacitor is used as a pixel of a display device or a capacitor of a nonvolatile memory cell. The upper electrode is configured to have a two-layered structure, for example. Specifically, an IrO2 film is formed on the ferroelectric film by using a sputtering process and then an Ir film is formed on the IrO2 film by also using a sputtering process.
In a process performed after forming the ferroelectric capacitor, the ferroelectric capacitor is often exposed to a reducing atmosphere. Specifically, when forming an interlayer dielectric film made of SiO2 by using a plasma CVD (Chemical Vapor Deposition) process, a semi-finished product including a ferroelectric capacitor is exposed to a reducing atmosphere containing a large amount of hydrogen and hydrogen radicals. In some cases, a miniaturized device may use a tungsten (W) plug having a high buriability, and such tungsten plug is formed under a reducing atmosphere. Specifically, when forming a tungsten plug that contacts with an upper electrode, by using a CVD process, a raw material gas including WF6 is reduced to its elements using hydrogen or silane. Accordingly, the semi-finished product including the ferroelectric capacitor is exposed to the reducing atmosphere.
Since a ferroelectric film is often made of a metal oxide such as PZT (lead zirconate titanate), characteristics of the ferroelectric film are deteriorated when it is exposed to the reducing atmosphere. Accordingly, there is provided a method to prevent the reducing atmosphere from being introduced into the ferroelectric film by forming a conductive hydrogen barrier film on the upper electrode.
In the above method, the conductive hydrogen barrier film is formed with an amorphous film made of IrTa. However, the use of such an additional amorphous film complicates the entire process. In particular, when etching the conductive hydrogen barrier film, the upper electrode, the ferroelectric film and the lower electrode into a common pattern, it may be difficult to obtain an appropriate etching selectivity between the conductive hydrogen barrier film and the ferroelectric film to form a fine pattern.
On the other hand, when omitting the conductive hydrogen barrier film, the upper electrode is readily exposed to the reducing atmosphere. If the upper electrode has a structure including an IrO2 film and an Ir film formed thereon by a sputtering process, the Ir film is grown into a columnar crystalline structure. Accordingly, the reducing atmosphere reaching the upper electrode continues to make contact with the IrO2 film through the grain boundary and reduces the IrO2 film. As a result, the IrO2 film loses oxygen therein and becomes a porous film mainly made of iridium metal. This may cause decrease in the contact area between the upper electrode and the ferroelectric film and deterioration of characteristics of the ferroelectric film due to the reducing atmosphere making contact with the ferroelectric film. Thus, the characteristics (particularly, capacitance) of the ferroelectric capacitor as required in the design cannot be provided.
The above problem has not been notable and considered when a capacitor area remains to be about 6 μm2 (2 μm×3 μm) or greater. However, such problem particularly becomes notable in devices having a capacitor area of equal to or less than 1 μm2 (for example, 0.85 μm2). Specifically, while measuring characteristics of trial-manufactured ferroelectric capacitors having an area of equal to or less than 1 μm2, the above-mentioned problem resulted in a failure of obtaining designed characteristics of the trial-manufactured ferroelectric capacitors.